This invention relates to the recovery of non-aqueous phase liquids (NAPL) from ground sources contaminated therewith, and in particular to a method and apparatus for such recovery, by injection of a gas-supersaturated liquid into the region of ground source of contamination, using a system of wells and a NAPL recovery system at the surface.
NAPL such as gasoline spills, halogenated solvents and other volatile petroleum products, are common groundwater contaminants. Dense NAPL can migrate below the water table and become trapped in the water-saturated zone, held in place by capillary forces. Since NAPL have very low solubility in water, the trapped NAPL can be a long term source of contamination of groundwater for many-many years. A schematic of trapped NAPL pool beneath the water table in an aquifer can be seen in FIG. 1.
Air sparging is a common remediation technique, seeking to capitalize on the relatively high NAPL volatility. However, this method is often very ineffective, primarily because the injected air tends to finger-through and by-pass most of the contaminated zone under the combined action of buoyancy and capillary forces. Air sparging is an immiscible displacement process and the capillary forces prevailing in the porous media will restrict gas flow through much of the pore space. As a result, the air-sparging method fails to achieve direct contact between most of the free-phase NAPL and the injected air, and its performance is severely compromised by mass transfer resistances in the aqueous phase. A schematic of this phenomenon is visualized in FIG. 2.
According to the invention disclosed in our prior U.S. Pat. No. 6,209,855 of 3 Apr. 2001, a method and apparatus are provided to produce a gas/liquid mixture which, when compared to known gas/liquid mixtures, has a) a surprisingly greater mass of gas dissolved in a given volume of liquid, to the point of super-saturation, and b) exhibits a vastly increased period during which gas remains dissolved in the liquid, before it nucleates and exits the liquid in the form of bubbles.